The invention described herein was made by an employee of the United States Government, and may be manufactured and used by or for the Government for governmental purposes without the payment of any royalties thereon or therefore.
This invention pertains to the art of methods and apparatuses for measuring strain in a specimen, and more specifically to methods and apparatuses for measuring the principal strain magnitudes and directions, and maximum shear strain that occurs within a specimen when subjected to strain.
The present invention contemplates a new and improved extensometer for measuring strain in a specimen, such as plastic, ceramic or porous metal, which is simple in design, effective in use, and overcomes the foregoing difficulties and others while providing better and more advantageous overall results.
The extensometer disclosed herein may also be used to measure strain in bone. Use of the extensometer for measuring in vivo bone strain is more fully described in U.S. Pat. No. 6,059,784, which is herein incorporated by reference.
Typically, surface-mounted strain gauges are used to measure the strain on the surface of a specimen. Unfortunately, surface mounted strain gauges do not work well on specimens having a high porosity, nor are they able to distinguish between axial strain and strain due to bending in a specimen. Surface mounted strain gauges are commonly arranged in a rosette to provide principal strains and their direction, and maximum shear strains within the plane of the gauge. The gauge or gauges must be bonded to the surface of the specimen, therefore the specimen must be compatible with the chemical bonding agents used to mount the gauge, be able to tolerate the surface preparation required to mount a strain gauge, and if bending in the specimen is to be measured, be able to allow multiple gauges to be mounted. Since some specimens may not be suitable for surface mounted gauges, a means to measure strain in these specimens is needed.
The present invention is directed to an extensometer, and a method of operation thereof, that measures pin displacement, from which strain is calculated, and from which, in a preferred embodiment, principal strain magnitudes and directions, maximum shear strain, and strains due to bending may be calculated via strain transformation relationships and geometric parameters of the extensometer.
The extensometer comprises at least two pins adapted to be inserted into the specimen; and at least two capacitive sensors mounted across the pins and providing a variable capacitance whose output is varied by the strain to which the specimen is subjected.
In one embodiment, six sensors are arranged into three pairs with each pair being attached to a mounting member having at least two (2) internal sides. The sensor pairs are oriented from each other by an angle dictated by the user of the device depending on the strain transformation relationships used. Preferably, however, the mounting member is a five-sided member having two internal sides and the sensor pairs are oriented from each other by an angle of 120 degrees.